Method and apparatus for improved centrifugal analysis



NOV. 4, 1969 M, JONES 3,475,968

METHOD AND APPARATUS FOR IMPROVED CENTRIFUGAL ANALYSIS Filed March 13, 1967 2 Sheets-Sheet 1 Nov. 4, 1969 JONES 3,475,968

METHOD AND APPARATUS FOR IMPROVED CENTRIFUGAL ANALYSIS Filed March 13, 1967 2 Sheets-Sheet 2 SAMPLER United States Patent 3,475,968 METHOD AND APPARATUS FOR IMPROVED CENTRIFUGAL ANALYSIS Malcolm H. Jones, Blaydon, England, assignor to Joyce, Loehl & Company Limited, Gateshead, England, a British company Filed Mar. 13, 1967, Ser. No. 622,624 Claims priority, application Great Britain, Mar. 18, 1966, 12,032/ 66 Int. Cl. Gtllf 15/14; G01j 5/04; G01k 1/08 US. Cl. 73-432 6 Claims ABSTRACT OF THE DISCLOSURE A method of centrifugal determination of the size of particles suspended in a mother liquid, in which a spin fluid is first introduced as a layer in a rotating centrifuge, followed by a thin layer of buffer fluid of a density slightly less than the spin fluid, producing a momentary change of speed of the centrifuge to partially mix the spin and buffer fluids, adding a layer of mother liquid containing particles of a range of sizes, allowing the particles to diffuse outwards for a predetermined time and then by sampling and/or observation to obtain an evaluation of the relative amounts of particles of different sizes.

My invention relates to a method and apparatus to effect an improved form of centrifugal analysis of the size of particles in a mother liquid.

The invention is more particularly concerned with a process of centrifugal analysis wherein a mother liquid containing dispersed particles is introduced on top of a spin fluid, the two being contained in a rotating centrifuge, the particles being driven outwards from the mother liquid and through the spin fluid at a rate determined by their particle size. The larger particles of course travel further into the spin fluid than the smaller particles for given centrifuge conditions. The particle size distribution is then obtained either by using photoextinction or a direct sampling technique. The extinction with respect to time, or cumulative weight of particles collected, are then made valid by solving Stokes equation for certain centifuge conditions.

Direct sampling or fractionation is preferred, because the assumptions correlating extinction to cumulative weight of particles, are suspect with photosedimentometry.

This method is known as the two-layer or line start technique, commonly used when particle size determination is to be effected by means of a disc centrifuge. It has however been known for some time that an unstable condition exists at the junction between the mother liquid and the spin fluid, when the density and viscosity relationships are theoretically correct for stable operation.

A barrier is apparently created at the liquid/liquid interface, this barrier breaking down at various discreet points to allow streams of particles to be emitted through them into the spin fluid instead of the particles spreading uniformly throughout the whole of the outside of the layer of the mother liquid into the spin fluid. This causes incorrect determination of particle size to be effected. The streaming effect can be seen clearly by the use of a stroboscope and is common to the majority of centrifuged particulate systems' The particle size determinations resulting from streaming depart appreciably from the values obtained when stable sedimentation is realised. Usually a more coarse particle size distribution is obtained with streaming.

I have found that this streaming effect is probably caused by the sudden change in density across the interface between the mother liquid and the spin fluid, and

that it can be avoided by removing the interface across which there is a density difference by tapering-off the density between these fluids in a thin buffer layer or layers between the two liquids.

According to my invention, a method of stabilising the size distribution of particles emanating from a mother liquid consists in the steps of introducing a spin fluid into a rotating disc centrifuge, applying a thin intermediate buffer layer or layers of fluid on the surface of the spin fluid of a density less than that of the spin fluid with partial mixing of the spin and intermediate fluids, applying a layer of mother liquid plus particles on the surface of the intermediate layer or layers, allowing the particles to diffuse under the action of centrifugal force through the intermediate and spin fluids for a predetermined time, and then by applying a standard measurement or sampling technique to interpret the sedimentation fields, and produce the particle size distribution curve.

According to an aspect of my invention, a centrifuge apparatus for effecting the method described comprises a disc centrifuge, means for accurately predetermining the speed of the centrifuge and maintaining it automatically at the predetermined speed, means for overriding the speed-maintaining means momentarily so as to provide a rapid positive or negative acceleration of short duration of the centrifuge, and means for injecting predetermined quantities of spin fluid, buffer fluid and mother liquid into the centrifuge, and stroboscope means for viewing the liquid to liquid interfaces, to enable accurate mixing of the spin fluid and buffer fluid interfaces to be effected.

In one method, the centrifuge is rotated, and a layer of spin fluid is introduced. The density of this spin fluid is usually greater than one for aqueous systems or less than one for non-aqueous systems. In any case, it is greater than the mean density of the mother liquid plus particles; this is to enable the mother liquid to remain at the top of the spin fluid during particle migration. On top of the spin fluid we introduce one or more very thin layers of buffer liquid of a density between that of the spin fluid and the mother liquid, so that a stepped or preferably continuous density gradient is formed between, the two fluids. The buffer liquid surface in the centrifuge should preferably be of the same nature as the mother liquid. The mother liquid plus particles is then introduced into the centrifuge, and being of similar nature and density to that of the buffer liquid it diffuses rapidly on injection, yet retains the particles in the mother liquid. The particles settle through the short density gradient into the spin fluid without being retarded and are centrifuged outwards into the spin fluid at a rate determined by Stokes predictions. By this means, a sudden change in density i.e., a density barrier with resulting hydrodynamic properties which effect particle sedimentation is reduced or prevented, so that streaming does not occur.

Reference should now be made to the accompanying drawings, in which:

FIGS. 1 to 4 are graphs illustrating the steps of our method.

FIG. 5 is a diagrammatic representation of an apparatus for carrying out the said method.

FIGURES 1 to 4 are graphs illustrating fluid density (D) petted against centrifuge radius (R). In the first step of my process (FIG. 1) I introduce a spin fluid S of a suitable density into the centrifuge and then introduce a thin buffer layer of fluid W of a lower density (L) preferably in the region of the density of the mother liquid itself or even consisting partly of the mother fluid. The centrifuge is then subjected to a short period of acceleration either positive or negative, causing a partial mixing of this second or intermediate layer with the spin fluid thus removing the liquid/liquid interface, so that there is no sudden change in density between the two (FIG. 2). The liquids in the centrifuge disc are allowed to stabilise. Then the mother liquid plus particles M is injected in the form of a thin layer on top of the intermediate fluid (FIG. 3). In practice interface (I) becomes diffused on injection of the dispersion. The density of the particles is shown as a line P. The particles very rapidly move across the new interface (I) across which there is no appreciable density ditference, into a region where the density (and viscosity) of the spin fluid is progressively increasing (FIG. 4). Because the length of the density gradient is short with respect to the depth of spin fluid, the sedimentation of particles can still be accurately predicted using the standard applicable Stokes equation, or for absolute accuracy of prediction a slightly modified equation could be produced to account for the small gradient. No streaming is produced, seeing that no density barrier exists between the mother liquid and the spin fluid.

I also provide apparatus in order to effect this particular method of operation. In this apparatus shown in FIG. 5, a centrifuge of the disc type 5 is spun at one of seven predetermined speeds between 1000 rpm. and 8000 rpm. by means of a motor 3 fed with current pulses of controlled width of amplitude, from a control circuit 2, the motor speed being compared with the frequency of a tuning fork oscillator 8 or one of its harmonics in the control circuit, the pulse length or amplitude of the driving current supplied to the motor at any instant being determined by the comparison between the motor speed and that of the said harmonic, so that the motor speed is maintained very accurately constant. The motor speed is conveyed to the control circuit by an electronic tachometer 4. The power line is represented by element 1. We also provide a push-button 7 whereby the synchronising circuit is overridden, so that the motor is momentarily fed with full power. This causes a sudden acceleration of the centrifuge disc, followed by a slow deceleration back to the chosen synchronous speed. During this sudden acceleration period, the interface produced by the intermediate layer which has been introduced on top of the spin fluid, mixes and is completely diffused with the spin fluid, so that no sudden change in density between the two fluids results. A conventional sampling device 10 can be used to remove specimens of the fiuid for analysis. This condithion is observed by means of a synchronised stroboscope 6. The predetermined quantities of spin fluid, buffer fluid and mother liquid are injected into the rotating centrifugal disc by a nozzle or other conventional apparatus as schematically represented by element 9. Preferably the nozzle injects the fluid at a flow speed approximating the speed of the centrifugal disc. The density of the intermediate layer is chosen as stated above so as to be similar or identical to that of the motor liquid, so that when the latter is introduced, again no high density barrier exists between the two fluids and the surface skin is broken on injection, so that the sedimentation of particles is stable and in agreement with theory.

Alternatively, the centrifuge may be provided with a brake whereby the speed is suddenly reduced by a predetermined amount, the speed then returning to normal slowly under the influence of the synchronising circuit.

In a particular application of my method, the spin fluid consists of a mixture of water and an inert substance such as glycerine or sugar (non-aqueous systems are also used) with concentrations adjusted to give a required density or viscosity.

Example 1 The intermediate or buffer layer is distilled water (or non-aqueous medium) and the mother liquid itself is also water (or non-aqueous medium) in which the particles to be measured are suspended. The particle size distribution is then measured by one of the standard accepted techniques of photoextinction or fractionation or sampling, with subsequent analysis.

Spin fluid20% weight/vol. glycerol and distilled water. Buffer layerl ml. distilled water.

Example 2 Mother liquid-1% weight/volume of red pigment 2 to distilled water, with 1 drop of Lissapol N particle density 1.4 gm./ml.

Spin fluid-20% weight/volume of glycerol to distilled water.

Buffer layer-l ml. distilled water.

Quantities and times as for Example 1.

Example 3 Motor liquid /2% weight/volume powdered titanium dioxide to distilled water, density 4.2 gm./ml. with 2% weight/volume 0.1% Calgon solution/dispersion and 1 drop of Lissapol N.

Spin fluid20% weight/volume glycerol to distilled water.

Buffer layer-2 ml. distilled water.

30 ml. of spin fluid injected into centrifuge, 2 ml. distilled Water added as buffer layer; centrifuge speeded up 10% and stabilised for 20 seconds. /2 ml. mother liquid added.

Total centrifuge time for seven particle sizes between 0.1 micron and 1 micron, 47 minutes.

I claim:

1. A method of stabilising the size distribution of particles emanating from a mother liquid, consisting of the steps of introducing a spin fluid into a rotating disc centrifuge, applying a thin intermediate buffer layer or fiuid on the surface of the spin fluid of a density less than that of the spin fluid with partial mixing of the spin and intermediate fluids, applying a layer of mother liquid plus particles on the surface of the intermediate layer, allowing the particles to diffuse under the action of centrifugal force through the intermediate and spin fluids for a predetermined time, and then by applying a standard measurement or sampling technique to interpret the sedimentation fields, and produce the particle size distribution curve.

2. A method as recited in claim 1 wherein the density of the material of which the particles are composed is higher than that of any of the fluids used.

3. A method as recited in claim 1 in which all the fluids mix with water.

4. A method as recited in claim 1 in which the density of the buffer layer fluid is less than that of the spin fluid, and the density of the fluid used in the mother liquid is not greater than the density of the buffer layer fluid.

5. Apparatus for stabilizing the size distribution of particles emanating from a mother liquid comprising, a rotating disc centrifuge, means for accurately predetermining the speed of the centrifuge and maintaining it automatically at the predetermined speed, means for overriding the speed maintaining means momentarily so as to provide a rapid positive or negative acceleration of short duration of the centrifuge, a first layer of spin fluid, a second laker of a thin intermediate buffer fluid on the surface of the spin fluid of a density less than that of the spin fluid, with partial mixing of the spin and intermediate fluids, a third layer of mother liquid plus particles on the surface of the second, means for injecting predetermined quantities of spin fluid, buffer fluid and mother liquid in sequence into the centrifuge, and sampling means 3,475,968 5 6 capable of removing a sample from a fluid layer, whereby References Cited the particles are diffused under the action of the centrifu- UNITED STATES PATENTS gal forces through the intermediate and spin fluid prior to removing a sample 2,956,434 10/1960 Donoghue 73-432 6. Apparatus for stabilizing the size distribution of 5 3237455 3/1966 Slater 73*432 particles as claimed in claim 5 and having stroboscope 3,243,106 3/1966 Atherton et a] 73 432 X means for viewing the liquid-to-liquid interfaces thus enabling accurate mixing of the spin fluid and the buffer LOUIS PRINCE Pnmary Exammer fluid interface to be effected. J. W. ROSKOS, Assistant Examiner 

